Human hair is comprised of the protein keratin, a hard, horny, fibrous material which consists of long, tapering fibrillar cells that have coalesced and contain about 16.6 to 18% of the amino acid cystine. The structure of the hair comprises an outer cuticle, a main portion called the cortex and often a central medulla.
The hair shaft or cortex is covered by a layer of thin, outer, colorless scales forming the cuticle. This layer of scales has the appearance of overlapping roof tiles with their free ends directed toward the tip of the hair. The main portion of the hair shaft, the cortex, comprises very closely packed longitudinally arranged spindle shaped cells and fibers firmly attached to each other. These cells and fibers contain air bubbles and pigment (melanin) that gives color to the hair. The medulla, if present, is a loose arrangement of axial fibers and angular cells containing air bubbles. The color of the hair is due to the pigment in the cortical cells and light reflected from the central medulla.
The protein of the hair consists of amino acids, linked peptide fashion with each other with the amino groups of one amino acid molecule linked to the carboxyl group of another amino acid molecule. However, this linkage can be broken by the action of concentrated solutions of strong alkalis or acids and also by certain enzymes. In addition, the keratin molecules are strengthened by so-called hydrogen linkages. When the hair is caused to swell, these bonds are weakened and are broken by certain concentrated salt solutions.
The weakening and/or destruction of these bonds is detrimental to the health and vitality of the hair. Alkalis and acids reduce or nullify the mechanical and chemical strength of this hair protein. In an alkaline medium, water causes a discernible swelling of the hair protein and the polypeptide chains are pushed apart. As the pH value rises higher into the alkaline range, the greater the degree of this swelling of the hair shaft. As a result, the scales of the cuticle laminae are pushed apart and may even become detached from the fibrous shaft. This action leads to further damage to the hair since alkali can now penetrate directly into the horny protein. Application of strong alkaline solutions at elevated temperatures can lead to the hair being dissolved. Moreover, hair having a pH in the alkaline range generally is dull and lusterless. Weak aqueous acids, while desirably closing the cuticular laminae of the hair to prevent undesirable penetration thereof, can also result in the formation of undesirable neutralization salts. Acids also split the salt-type cross linkage found in the hair. Strong acids damage the hair and stronger acids, below pH 2, destroy hair. Additionally weathering effects, i.e. damage to hair by environmental factors, such as by sunlight, sea water, chlorinated pool water and air pollutants, such as acid rain, chemically alter hair, which alterations can be detected at the morphological level.
These effects have provided the basis for many treatments of the hair. That is, the opening of the cuticle scales and exposure of the cortex by alkalis has enabled the formulation of various alkaline preparations for various treatments of hair. For example, bleaching preparations of pH 9.5 to 10; hair tints or dyes of pH 8 to 1; waving preparations of pH 8 to 10; hair straighteners of pH 11 to 12; and shampoos or shaving soaps of pH 8 to 10 have been formulated to take advantage of that phenomena. Similarly, acid preparations such as fixatives or neutralizers of pH 2.5 to 4.0; peroxide hair lighteners of pH 2.5 to 3.0; hair setting agents of pH 4.5 to 7.0 and shampoos of pH 5 to 7 have been formulated to take advantage of the effect of acid solution on hair.
However, hair has greater strength and resiliency when in its isoionic region, i.e. when at a pH of from about 6.0 to 7.0; and its greatest strength and resiliency when at or near its isoionic point of about pH 6.4--the isoionic point reported by Steinhardt and Harris, J. Res. Nat. Bur. Stand. 24, 335-367 (1940). The pH at which a protein or a particle has an equivalent number of positive and negative charges which arise exclusively from proton exchange is the isoionic point. The isoionic point is a whole-fiber property of hair and is reflected by the equilibrium acid-base properties of the total hair fiber. For long-term interactions, if the pH of a surrounding solution is below the isoionic point of hair, the hair will pick up acid, and above its isoionic point, it will attract hydroxide ions more readily. The isoionic point therefore becomes more important to whole-fiber treatments such as perms, bleaches, straighteners and hair dyes. When outside the isoionic range, hair is easily broken upon combing, brushing or teasing or other similar treatments. The cuticles have an increased ability to withstand wear when hair is in the isoionic range and thus hair is able to grow longer due to fewer loss of cuticles.
The various alkali and acid formulations employed to obtain various effects upon the hair, such as those mentioned hereinbefore, have led to hair often being outside its isoionic range, i.e. either in the alkaline range above or the acid range below the isoionic range. Buffered rinses or conditioners have been utilized to treat such alkaline or acid pH hair in an attempt to bring the hair pH back to or close to its isoionic range. However, the use of such rinses or conditioners have been on a chance or guesswork basis, i.e. by rinsing with a buffered acid rinse or conditioner after undergoing an alkali treatment such as bleaching or tinting, or by rinsing with a buffered alkali rinse or conditioner after undergoing an acid treatment such as peroxide hair lighteners. However, such chance treatment may or may not bring the hair back to its desired isoionic range.
It is therefore highly desirable that a formulation and process be available to rapidly and easily determine if hair is within its isoionic range or outside said range, and if outside the range whether outside on the acid or alkaline side. This would enable one to knowingly use the appropriate buffered rinse or conditioner to restore hair to within its isoionic range and preferably to the isoionic point of about pH 6.4.